Coated composite high voltage electrical insulator

ABSTRACT

The present invention relates to polymer based composition high voltage insulator having a monolithic coating of a one-part room temperature vulcanizable organopolysiloxane rubber composition which crosslinks in the presence of moisture. The one-part organopolysiloxane rubber composition comprises the product which is obtained by mixing from about 20 to about 60 weight percent of one or more polydiorganosiloxane fluids, from 0 to about 40 weight percent of a cyclo-organosiloxane, of the formula [(R) 2 SiO] n  from 0 to about 40 weight percent of an inorganic extending or non-reinforcing filler, from about 0.5 to about 15 weight percent of an amorphous SiO 2  reinforcing filler having a surface area of between about 100 to 250 m 2 /g and a particle size range between about 0.01 and 0.03 microns from about 1 to about 10 weight percent of an oximinosilane cross-linking agent.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of PCT/CA02/01920 filed Dec.16, 2002, which is a continuation-in-part of Ser. No. 10/014,790, filedDec. 14, 2001, now abandoned both pending.

FIELD OF THE INVENTION

This invention relates to a method for refurbishing composite highvoltage electrical insulators by coating the entire surface of theinsulator with a monolithic one-part room temperature vulcanizableorganopolysiloxane rubber composition that does not contain volatileorganic compounds (VOC) and which crosslinks in the presence of moistureto form a coating to protect the composite high voltage insulators fromenvironmental effects.

BACKGROUND OF THE INVENTION

Electrical insulators for high voltage use have traditionally been madeof glass or porcelain as these materials are of low cost, high qualityelectrically and under normal conditions, have a long life in service.However, at higher voltages, the size and weight of glass or porcelainbecomes excessive. A number of composite insulators made from lighterweight polymeric materials have been developed for use in such highvoltage installations. Such composite insulators generally include afiberglas rod having a number of weathersheds constructed of a highlyinsulating polymeric material attached to the rod along its length.

Since the 1970's, advancements in high voltage insulator technology havebeen made by introducing new, lightweight polymer materials for themanufacture of insulators-replacing the heavier and more brittleporcelain and glass materials conventionally used in the manufacture ofinsulators. These new polymer materials include Silicone, EPDM, ESP,EPR, Bisphenol Epoxy and Cycloaliphatic Epoxy polymers.

Although they offer the advantages of reduced weight and mechanicalflexibility, insulators made from any of these polymers havedemonstrated failure and inconsistent performance when subjected tonormal field phenomena such as environmental weathering, UV exposure,exposure to industrial pollution, exposure to salt fog and salt spray,electrical tracking, corona discharge and electrical arcing. Unlike thisnew technology, the older porcelain/glass technology is extremelyreliable. Insulators made of porcelain/glass have a minimum lifespan of35 years compared to inconsistent lifespans of only 2 to 20 years withpolymer-based composite insulators.

The primary modes of failure of polymer-based composite insulators areerosion of the polymer material, splitting/cracking of the polymermaterial and brittle fracture of the supporting fiberglass core (due tothe interface of dissimilar materials; polymer and fiberglass). Loss ofhydrophobicity is also a common mode of failure, particularly ininsulators made of non-silicone polymers. These non-siliconepolymer-based composite insulators have thus not gained much acceptancein the market, despite their substantially lower cost compared tosilicone polymer-based composite insulators and porcelain/glassinsulators.

An insulator such as the suspension insulator in a high voltage powertransmission line is designed to keep to a minimum current dischargesunder normal conditions. However, when the insulator surface becomescontaminated, leakage current can develop along the surface of theinsulator. The amount of this leakage current depends upon the voltagestress and the conductivity of the film of contaminant on the surface ofthe insulator. These leakage currents can incur or cause arcing on thesurface of the insulator which can have serious effects upon thecomposite insulator surface such as the formation of free carbon and nonvolatile semi-conductor materials and may eventually result in aconducting path forming across the surface of the insulator effectivelyshorting out the insulator.

The outer surface of an electrical insulator is the most important partof the insulator as this is the part of the insulator that is subjectedto the effects of electrical voltage stress, leakage currents andweathering. When the surface of a high voltage insulator is exposed tomoisture such as rain or fog in combination with contaminatedatmospheres as are found in industrial locations may be subject toextensive corrosion unless protected in some way from exposure to thecorrosive atmosphere. Other potentially corrosive environments includealong sea coasts where salt spray is found and in areas whereagricultural chemicals are widely distributed.

The room temperature curable silicone composition of the presentinvention used to coat the outer surface of insulators provides forimproved insulation that is arc resistant, hydrophobic and resistant tothe stresses imposed upon outdoor electrical insulator. The compositionprovides a coating of electrically non conductive material on thesurface of the insulator which protects the underlying insulatormaterial.

SUMMARY OF THE INVENTION

The present invention provides a VOC free, one part room temperaturevulcanizable coating for protecting composite insulators. The coatingprovides for easy and convenient application by conventional methodssuch as dipping, flow or spraying. The coating provides a guard againstenvironmental effects along with high physical strength and adhesionachieved with a suitable blend of reinforcing and extending fillers.

In one aspect, the present invention provides for a one-part roomtemperature vulcanizing organopolysiloxane rubber composition forcoating a composite high voltage insulator. The composition comprisesthe product which is obtained by mixing the following:

-   -   a) from about 20 to about 60 weight percent of one or more        polydiorganosiloxane fluids of the formula        R″[(R)₂SiO]_(n)(R)₂SiR′        -   in which R is a monovalent alkyl or alkenyl radical having 1            to 8 carbon atoms or a phenyl radical, R′ and R″ which may            be the same or different are OH or a monovalent alkyl or            alkenyl radical having 1 to 8 carbon atoms or a phenyl            radical and n has an average value such that the viscosity            is from about 10 to about 100,000 centipoise at 25° C.,            wherein at least one of the polyorganosiloxane fluids has at            least one of R′ and R″ equal to OH and n has an average            value such that the viscosity is in the range from 100 to            100,000 centipoise at 25° C., preferably from 1,000 to            40,000 centipoise at 25° C.;    -   b) from 0 to about 40 weight percent of a cyclo-organosiloxane        of the formula        [(R)₂SiO]_(n)        -   in which R is a monovalent alkyl or alkenyl radical having 1            to 8 carbon atoms or a phenyl radical which may optionally            be substituted with an alkyl radical having 1 to 8 carbon            atoms and n has an average value of 3 to 10    -   c) from 0 to about 40 weight percent of an inorganic extending        or non-reinforcing filler    -   d) from about 0.5 to about 15 weight percent of an amorphous        SiO₂ reinforcing filler having a surface area of between about        100 to 250 m²/g and a particle size range between about 0.01 and        0.03 microns;        -   the surface of amorphous silica may also be treated with            organic molecules such as hexamethyldisilazane or            polydimethylsiloxane or silane;    -   e) from about 1 to about 10 weight percent of an oximinosilane        cross-linking agent of the formula;        RSi(ON═CR′₂)₃        -   in which R and R′ are independently selected from monovalent            alkyl or alkenyl radicals having 1 to 8 carbon atoms or a            phenyl radical        -   which may optionally be substituted with an alkyl radical            having 1 to 8 carbon atoms;    -   f) from about 0.2 to about 3 weight percent of an adhesion        promoter of the formula        -   in which R² and R³ are independently selected from            monovalent alkyl or alkenyl radicals having 1 to 8 carbon            atoms or a phenyl radical which may optionally be            substituted with an alkyl radical having 1 to 8 carbon            atoms, b is an integer between 0 and 3, and R¹ is a            saturated, unsaturated or aromatic hydrocarbon radical            having 1 to 10 carbon atoms which may optionally contain a            functional group;    -   g) from about 0.02 to about 3 weight percent of an organotin        salt as a condensation catalyst; and    -   h) from about 20 to about 50 weight percent of alumina        trihydrate, the alumina trihydrate having a median particle size        of about 10 to about 30 mm, containing 65.1 percent Al₂O₃, 34.5        percent combined H₂O, 0.3 percent Na₂O, 0.02 percent CaO, 0.01        percent SiO₂ and having a specific gravity of 2.42.

The present invention also provides for a method of protecting highvoltage composition insulators from environmental effects. The methodcomprises applying to the surface of the insulator a thin layer of theabove one-part organopolysiloxane rubber composition and allowing thelayer of the one-part organopolysiloxane rubber composition to cure atroom temperature to a silicone elastomer.

DETAILED DESCRIPTION OF THE INVENTION

The dilemma of trading off product lifespan and reliability forlightweight insulators is resolved by forming a monolithic layer ofsilicone rubber around the polymer-based composite insulator from top tobottom, encapsulating the entire insulator and overlapping the metalfittings at each end of the insulator. The protective layer may beformed of High Temperature vulcanizable (HTV) silicone rubber, liquidsilicone rubber, 2-part silicone rubber, catalyst cure silicone rubber,RTV silicone rubber or UV cure silicone rubber. In the preferredembodiment, the protective layer is a one-part RTV silicone rubber.

Because it is made of silicone, the resulting smooth layer around theinsulator provides protection against the otherwise damaging effects ofenvironmental weathering, UV exposure, hydrolysis, electrical tracking,corona discharge and electrical arcing. The resulting layer also acts toconceal any interface of the dissimilar polymer and fiberglass materialsand seals out the ingress of moisture between the polymer and itssupporting fiberglass core. Because of its naturally hydrophobic nature,the external layer of silicone creates a highly hydrophobic insulator ofvery low cost. The resulting insulator is able to withstand well above1,000 hours of accelerated testing under test method TEC 1109,equivalent to well over 10 years of actual field performance.

Although not so limited, the external silicone layer around the polymerinsulator is ideally formed with a low viscosity RTV, HTV or UV curesilicone coating to be applied by spray, brush or dip.

The one-part organopolysiloxane rubber compositions of the preferredembodiment of the present invention are ideally suited for refurbishingcomposite high voltage electrical insulators to provide protection ofthe refurbished composite high voltage insulators against corrosion andleakage current cause by the effects of salt spray and chemicalenvironments including direct exposure to salt water, salt fog, gasesand other industrial pollutants. The protection afforded by thecompositions of the present invention has surprising been found toprovide equivalent or in some cases better insulating properties ascompared to new composite insulators. The compositions with suitableadditives also provide protection against the effects of weathering fromexposure to among others UV radiation. The absence of volatile organiccomponents (VOC) in the coating composition makes it environmentallyfriendly, when large outdoor installations considered such as powertransmission lines etc.

The one-part organopolysiloxane rubber compositions of the presentinvention for use as a protective coating contain about 20 to about 60weight percent of one or more polydimethylsiloxane fluids of theformula:R″⊖[(R)₂SiO]_(n)(R)₂SiR′in which R is a monovalent alkyl or alkenyl radical having 1 to 8 carbonatoms or a phenyl radical, R′ and R″ which may be the same or differentare OH or a monovalent alkyl or alkenyl radical having 1 to 8 carbonatoms or a phenyl radical and n has an average value such that theviscosity is from about 10 to about 100,000 centipoise at 25° C. Atleast one of the polyorganosiloxane fluids is a higher viscositysiloxane having reaction groups in which at least one and preferablyboth of R′ and R″ is equal to OH and n has an average value such thatthe viscosity is in the range from 100 to 100,000 centipoise at 25° C.,preferably from 1,000 to 40,000 centipoise at 25° C. Thepolydimethylsiloxanes may contain small amounts of monomethylsiloxaneunits and methyl radical replaced with other radicals in small amountsas impurities such as is found in commercial products, but the preferredfluid contains only polydimethylsiloxane.

The composition may contain a second linear dimethyl polysiloxane of lowmolecular weight to act as a viscosity reducer for the composition forease in applying the composition to the surface. The low molecularweight linear dimethyl polysiloxanes are end blocked oligomericcompounds of the above formula where R, R′ and R″ may be the same ordifferent and are independently selected from a monovalent alkyl oralkenyl radical having 1 to 8 carbon atoms or phenyl radical. Theaverage value of n ranges between 4 and 24, preferably between 4 and 20.

If the composition contains the two different polysiloxanes set outabove, the total of the polysiloxanes is generally about 40 to 60 weightpercent with the relative amounts of the two polysiloxanes beingselected based upon the desired characteristics of the final coating.Generally each of the polysiloxanes will be present in a ratio of fromabout 30 weight percent to about 70 weight percent based upon the totalweight of the polysiloxane fluids.

In addition to, or in place of the low molecular weight linear dimethylpolysiloxanes, the composition may contain up to about 40 weightpercent, more preferably 20 to 30 weight percent of acyclo-organosiloxane of the formula:[(R)₂SiO]_(n)in which R is a monovalent alkyl or alkenyl radical having 1 to 8 carbonatoms or a phenyl radical which may optionally be substituted with analkyl radical having 1 to 8 carbon atoms and n has an average value of 3to 10. The preferred cycloorganosiloxane is a cyclic dimethylsiloxaneand is used in a similar manner to the low molecular weight lineardimethyl polysiloxanes to lower the viscosity of the composition forconvenient application by spraying, brushing or dipping.

The composition also contains about 20 to about 50 weight percent ofalumina trihydrate preferably about 35 to about 50 weight percent, thealumina trihydrate having a median particle size of about 10 μm to about30 μm, preferably about 10 μm to about 15 μm containing 65.1 percentAl₂O₃, 34.5 percent combined H₂O, 0.3 percent Na₂O, 0.02 percent CaO,0.01 percent SiO₂ and having a specific gravity of 2.42. The amount offiller may be increased within the range to improve the desiredproperties.

The composition also contains about 0.5 to 15 weight percent of anamorphous SiO₂ reinforcing filler having a surface area of between about100 to 250 m²/g and a particle size range between about 0.01 and 0.03microns. The specific gravity of the filler is preferably about 2.2. Thesurface of amorphous silica may also be treated with organic moleculessuch as hexamethyldisilazane or polydimethylsiloxane or silane.

The composition also contains about 1 to 10 weight percent, preferably 2to 5 weight percent of an oximinosilane cross linking agent. Preferablythe oximinosilane cross linking agent is of the formula RSi(ON═CR′₂)₃ inwhich R and R′ each represent a monovalent alkyl or alkenyl radicalhaving 1 to 8 carbon atoms or a phenyl radical, preferably an alkylradical such as methyl, ethyl, propyl, butyl, or an alkenyl radical suchas vinyl, allyl, or a phenyl radical. The preferred R and R′ are alkylor vinyl radicals, most preferably methyl and ethyl radicals.

The composition also contains about 0.2 to 3 weight percent of an organofunctional silane as an adhesion promoter. Preferably the organofunctional silane has the formula

wherein R² and R³ are independently selected from monovalent alkyl oralkenyl radicals being 1 to 8 carbon atoms or a phenyl radical, b is aninteger from 0 to 3, preferably 0, and R¹ is a saturated, unsaturated oraromatic hydrocarbon radical being 1 to 10 carbon atoms, which may befurther functionalized by a member selected from the group consisting ofamino, ether, epoxy, isocyanate, cyano, acryloxy and acyloxy andcombinations thereof. R² and R³ are preferably an alkyl radical such as,for example, methyl, ethyl, propyl, butyl, or an alkenyl radical such asvinyl and allyl. More preferably R² and R³ are alkyl radicals, mostpreferably methyl, ethyl or propyl radicals. Preferably R¹ is an alkylgroup, more preferably further functionalized by one or more aminogroup. The most preferred organo-functional silane isN-(2-aminoethyl-3-aminopropyl)trimethylsilane.

In all of the above compounds, the alkyl includes straight, branched orcyclic radicals. Among the alkyl groups are C₁₋₈ straight orbranched-chain alkyl such as, for example, methyl, ethyl, propyl,isopropyl, n-butyl, isobutyl, tert-butyl, pentyl, isopentyl, hexyl,etc., the cycloalkyl are C₃₋₈ cycloalkyl such as, for example,cyclopropyl, cyclobutyl, cydohexyl, etc., the alkenyl groups are C₁₋₈alkenyl such as, for example, vinyl and allyl. The above groups a wellas the phenyl radicals may be further functionalized by including in thechain or ring structure, as the case may be, a group selected from theclass consisting of amino, ether, epoxy, isocyanate, cyano, acryloxy,acyloxy and combinations, so long as the functionalization does notadversely affect the desired properties of the compound.

The composition additionally contains about 0.02 to 3 weight percent ofan organotin salt of a carboxylic acid as a condensation catalyst, whichaccelerate the aging of the composition. Preferably the organotin saltis selected from the group dibutyltin diacetate, stannous octoate,dibutyltin dioctoate and dibutyltin dilaurate. Most preferably theorganotin salt is dibutyltin dilaurate of the formula:(C₄H₉)₂Sn(OCOC₁₀H₂₀CH₃)₂.

The composition may contain other optional ingredients such as pigmentsand other fillers in minor amounts provided that the addition of theingredients does not cause degradation of the desired properties of thecured coating made from the composition. One commonly utilized optionalingredient is a pigment, preferably a gray pigment, most preferablypresent in amounts up to about 1 weight percent.

The organopolysiloxane composition of the present invention is preparedby mixing the ingredients together in the absence of moisture. Thesilane is moisture sensitive and will undergo cross-linking in thepresence of moisture such that the mixture must be essentially absent offree moisture when the silane is added and maintained in a moisture freestate until cure is desired.

A preferred method of mixing comprises mixing the polysiloxane polymerfluids with the reinforcing filler. Thereafter, the organosiliconecross-linking agent and the adhesion promoter are added under a nitrogenatmosphere. The alumina trihydrate is thereafter added in stages andmixed, and the solvent added to the mixture under a nitrogen atmosphere.Finally, the condensation catalyst is added to the mixture.

The improved refurbished coated composite insulator of the presentinvention is capable of operating under a higher voltage stress in thepresence of moisture such as rain or fog in combination withcontaminated atmospheres such are as found in industrial locations,along sea coast where salt spray is found and in areas whereagricultural chemicals are widely distributed.

The surface to be protected is coated with the composition byconventional methods such as dipping, brushing or spraying. Preferably,the surface to be protected is coated by spraying one or moreapplications of the composition of the present invention. Thecomposition may be adjusted to the consistency suitable for use in thesemethods by the adjustment of the amount of low molecular weightpolysiloxane viscosity reducer utilized in the composition or may beheated for spray application. The thickness of the coating will dependupon the specific requirements of the application and the desired levelof protection. The coating generally has an average thickness of 250 to1500 microns more preferably, an average thickness of 200 to 1000microns, most preferably about 200 to 750 microns. After the coating isformed on the surface, the surface is exposed to normal atmosphere forcross-linking and cure of the coating.

The following examples are included to illustrate preferred embodimentsof the invention and to demonstrate the usefulness of the coating andare not intended to limit in any way the scope of protection for theinvention.

EXAMPLE 1

A first composition for coating electrical insulators was prepared bymixing 30 parts by weight of dimethyl polysiloxane fluid having aviscosity of 20,000 centipoise at 25° C. and 20 parts by weight ofcyclic dimethyl siloxane then adding 2 parts by weight of surfacetreated amorphous silica (surface treated with hexamethyldisilazane)having a specific gravity of 2.2 and surface area of about 150 m²/g.Then 2 parts by weight of methyl tris-(methyl ethyl ketoxime)silane and1 part by weight of N-(2-aminoethyl-3 aminopropyl)trimethoxy silane areadded and mixed under nitrogen atmosphere. Then 35 parts by weight ofalumina trihydrate are added and mixed well to uniform consistency. Theviscosity of the mixture was checked and adjusted to 1000±300 centipoiseby adding extra amount of dimethoxy polysiloxane cyclics and amorphoussilica respectively. Finally, 0.1 part by weight of dibutyltin dilaurateis added and mixed thoroughly.

EXAMPLE 2

A second electrical insulator coating composition was prepared by mixing30 parts by weight of dimethyl polysiloxane fluid having a viscosity of20,000 centipoise at 25° C. and 29 parts by weight of linear dimethylpolysiloxane of viscosity of 50 centipoise at 25° C., then adding 2parts by weight of surface treated amorphous silica (surface treatedwith hexamethyldisilazane) having a specific gravity of 2.2 and surfacearea of about 150 m²/g. Then 2 parts by weight of methyl tris-(methylethyl ketoxime)silane and 1 part by weight of N-(2-aminoethyl-3aminopropyl)trimethoxy silane are added and mixed under nitrogenatmosphere. Then 35 parts by weight of alumina trihydrate are added andmixed well to uniform consistency. The viscosity of the mixture waschecked and adjusted to 1000±300 centipoise by adding extra amount ofdimethoxy polysiloxane linears and amorphous silica respectively.Finally, 0.1 part by weight of dibutyltin dilaurate is added and mixedthoroughly.

EXAMPLE 3

A solventless coating composition was prepared by mixing 46 parts byweight of polydimethylsiloxane fluid having viscosity of 2,000centipoise and 4 parts by weight of surface treated amorphous silica(surface treated with hexamethyldisilazane) having specific gravity of2.2 and surface area of about 130 m²/g. Then 6 parts by weight of methyltris-(methyl ethyl ketoxime)silane and 1 part by weight ofN-(2-aminoethyl-3-aminopropyl)trimethoxy silane are added and mixedunder nitrogen atmosphere. Then 40 parts by weight of alumina trihydratewere also added and mixed. To prepare a coating with desired colour 3parts by weight of pigment paste were also added and mixed to a uniformconsistency. The pigment paste was prepared by mixing 50 parts by weightof pigment powder into polydimethylsiloxane fluid. Finally 0.1 parts byweight of dibutyltin dilaurate is added and mixed thoroughly.The finished product has a viscosity that makes it suitable forapplication by roller and brush. The coating can also be applied byspray application by heating in situ to a temperature of 70° C.

The composition prepared in accordance with Example 1 was coated on anumber of 2 inch by 5 inch by 3/16 inch thick samples of GPO3 barriermaterial by dipping the samples to obtain an average coating thicknessof 0.5±0.05 mm. One coated sample was exposed to 3000 hours UV light andcompared to a control sample. There was only a very slight change incolor of the sample exposed to UV light compared to the control sample,indicating good UV resistance.

Six samples of GPO3 barrier material coated as above were tested forinclined plane tracking in accordance with test method ASTM D2303. Onesample tracked one inch at 880 minutes. Another sample started trackingat 920 minutes and had tracked about ¼ inch at 1000 minutes. The otherfour samples passed 1000 minutes with no tracking. Six control bare GPO3samples were also tested and all six displayed tracking or slighterosion by 1000 minutes. The coating demonstrates superior resistance totracking.

Four samples of GPO3 material coated as above were tested for arcresistance and all four samples passed the test of a minimum of 180seconds.

A 25 kV insulator including 200 amp and 600 amp bushings was coated witha composition prepared in accordance with Example 1 and compared to anuncoated insulator and a vapor blasted insulator in a salt fog chambertest in accordance with IEC 507 testing method. During a test in whichthe salinity level was changed until acceptable results were achieved,the coated sample attained a salinity level almost three times that ofthe uncoated sample. The peak leakage current for the coated snmpleduring the test was 16 mA versus 40 mA for the uncoated sample and 80 mAfor the vapor blasted sample.

A 25 kV insulator including 200 amp and 600 amp bushings was coated witha composition prepared in accordance with Example 1 and compared to anuncoated insulator in a tracking wheel test. There was no appreciabledifference in performance between the coated and uncoated insulators.

The compositions of the present invention are useful in many instanceswhere protection of surfaces of composite insulators againstenvironmental effects is desired. These compositions include thecompositions of the above examples as well as other compositions, theformulation of which is well within the skill of the ordinary workman inthe art. The selection of the various components and their proportionswould be immediately apparent depending upon the desired properties ofthe final coating. The compositions of the present invention overcomemany of the problems associated with prior art compositions being a onepart coating composition, which is easy to apply using any of thecommonly employed methods and which is VOC free, thus satisfying theelimination or reduction in environmental pollution and potential dangerto the health of workers caused by VOC containing compositions.

This invention relates to the use of a silicone mould or coating to forman exterior monolithic layer on a polymer-based composite high voltageinsulator in order to reduce the negative and destructive effects on theinsulator from phenomena such as environmental weathering, UV exposure,hydrolysis, electrical tracking, erosion of the polymer/brittle fractureof the composite insulator due to corona discharges and electricalarcing. Further, the exterior silicone layer works to increase theinsulator's effective lifespan by improving its hydrophobiccharacteristics and suppression of leakage current.

The invention is effective in use with any type/geometry of all types ofpolymer-based composite insulators made of Silicone, EPDM (EthylenePropylene Diene Monomer), ESP, EPR, Bisphenol Epoxy or CycloaliphaticEpoxy type polymers.

Although not so limited, the invention provides a highly resilient, lowcost high voltage insulator with a long lifespan for use on electricaltransmission and distribution lines, as demanded, but currentlyunavailable, by users of such equipment.

While the invention has been described in reference to specificembodiments it should be understood by those skilled in the art thatvarious changes can be made and equivalents may be substituted withoutdeparting from the true spirit and scope of the invention. All suchmodifications are intended to be within the scope of the claims appendedhereto.

1. A method of protecting a composite high voltage insulators from theeffects of a corrosive atmospheric environment comprising (1) applyingto the surface of the composite insulator a thin layer of a one-partorganopolysiloxane rubber composition comprising the product which isobtained by mixing the following: a) from about 20 to about 60 weightpercent of one or more polydiorganosiloxane fluids of the formulaR″[(R)₂SiO]_(n)(R)₂SiR′ in which R is a monovalent alkyl or alkenylradical having 1 to 8 carbon atoms or a phenyl radical, R′ and R″ whichmay be the same or different are OH or a monovalent alkyl or alkenylradical having 1 to 8 carbon atoms or a phenyl radical and n has anaverage value such that the viscosity is from about 10 to about 100,000centipoise at 25° C., wherein at least one of the polyorganosiloxanefluid has at least one of R′ and R″ equal to OH and n has an averagevalue such that the viscosity is in the range from 100 to 100,000centipoise at 25° C. b) from 0 to about 40 weight percent of acyclo-organosiloxane of the formula[(R)₂SiO]_(n) in which R is a monovalent alkyl or alkenyl radical having1 to 8 carbon atoms or a phenyl radical which may optionally besubstituted with an alkyl radical having 1 to 8 carbon atoms and n hasan average value of 3 to 10 c) from 0 to about 40 weight percent of aninorganic extending or non-reinforcing filler d) from about 0.5 to about15 weight percent of an amorphous SiO₂ reinforcing filler having asurface area of between about 100 to 250 m²/g and a particle size rangebetween about 0.01 and 0.03 microns; e) from about 1 to about 10 weightpercent of an oximinosilane cross-linking agent of the formula;RSi(ON═CR′₂)₃ in which R and R′ are independently selected frommonovalent alkyl or alkenyl radicals having 1 to 8 carbon atoms or aphenyl radical which may optionally be substituted with an alkyl radicalhaving 1 to 8 carbon atoms; f) from about 0.2 to about 3 weight percentof an adhesion promoter of the formula

in which R² and R³ are independently selected from monovalent alkyl oralkenyl radicals having 1 to 8 carbon atoms or a phenyl radical whichmay optionally be substituted with an alkyl radical having 1 to 8 carbonatoms, b is an integer between 0 and 3, and R¹ is a saturated,unsaturated or aromatic hydrocarbon radical having 1 to 10 carbon atomswhich may optionally contain a functional group; g) from about 0.02 toabout 3 weight percent of an organotin salt as a condensation catalyst;and h) from about 20 to about 50 weight percent of alumina trihydrate,the alumina trihydrate having a median particle size of about 10 μm toabout 30 μm, containing 65.1 weight percent Al₂O₃, 34.5 weight percentcombined H₂O, 0.3 weight percent Na₂O, 0.02 weight percent CaO, 0.01weight percent SiO₂ and having a specific gravity of 2.42.
 2. A methodaccording to claim 1 wherein the one part organopolysiloxane rubbercomposition comprises: a) about 30 weight percent of a hydroxylterminated dimethyl polysiloxane fluid having a viscosity of 10,000 to20,000 Centipoise at 25° C.; b) about 2 weight percent of a mixture ofamorphous and crystalline SiO₂ fillers having a specific gravity of 2.2and surface area of up to about 130 m²/g; c) about 2 weight percent ofmethyl tris-(methyl ethyl ketoxime)silane; d) about 1 weight percent ofN-(2 aminoethyl-3 aminopropyl)trimethoxysilane; e) about 0.1 weightpercent of dibutyltindilaurate; f) about 29 weight percent ofdimethylsiloxane cyclics; and g) about 0.8 weight percent of a graypigment; h) about 35 weight percent of alumina trihydrate, the aluminatrihydrate having a median particle size of 13 μm, containing 65.1weight percent Al₂O₃, 34.5 weight percent combined H₂O, 0.3 weightpercent Na₂O, 0.02 weight percent CaO, 0.01 weight percent SiO₂ andhaving a specific gravity of 2.42.
 3. A method according to claim 1wherein n has an average value such that the viscosity is in the rangefrom 1,000 to 40,000 centipoise at 25° C.
 4. A method according to claim3 wherein R is methyl.
 5. A method according to claim 4 wherein theadhesion promoter is a compound of the formula

wherein Me is a methyl radical.
 6. A method according to claim 5 whereinthe organotin salt is an organotin salt of a carboxylic acid selectedfrom the group consisting of dibutyltindiacetate, stannous octoate anddibutyltindilaurate.
 7. A method according to claim 6 wherein theorganotin salt of a carboxylic acid is a compound of the formula(C₄H₉)₂Sn(OCOC₁₀H₂₀CH₃)_(2.)
 8. A method according to claim 7 whereinthe composition contains a first polydiorganosiloxane fluid wherein R isalkyl, R′ and R′ are each OH and n has an average value such that theviscosity is from about 1000 to 30,000 centipoise at 25° C. and a secondpolydiorganosiloxane fluid wherein R, R′ and R″ are each alkyl and theaverage value of n is between 4 and
 20. 9. A method according to claim 8wherein R of the first polydiorganosiloxane and R, R′ and R″ of thesecond polydiorganosiloxane are all methyl.